1. Field of the Invention
The present invention relates generally to the field of waste water treatment and more particularly to a method of cleaning waste water containing contaminants and the recovery of such contaminants therefrom. The present invention is applicable to a broad range of contaminants, but has particular application to the clean up of waste water from coating processes, and in particular, oil based coating processes such as lithographic or other printing processes, paint or varnish application processes or the like in which the oil based coating composition is water insoluble at first pH levels and water soluble or washable at second pH levels.
2. Summary of the Prior Art
Water treatment in general has grown significantly over the past several years. Treatment methods which result in clean water with minimal chemical or energy input are highly desirable. This is particularly true with industrial waste water effluents containing contaminants resulting from the clean up of various coating processes including lithographic or other ink printing processes and paint or varnish application processes. Waste water effluents from such processes often contain organic wash solvents which are needed to clean up application or processing equipment and tools. The laundering of shop towels and other cleaning aids containing such contaminants and organic wash solvents are also a source of pollution since the laundering medium containing such contaminants is commonly disposed of into the sewer system.
In a lithographic printing process which utilizes both an oil based ink composition and an aqueous fountain solution, printing plates and/or inks need to be periodically changed as one job is completed and another started. Whenever this occurs, the blanket cylinder and/or the entire print train including the application rollers, the print plate, etc. must be cleaned. Such cleaning is commonly accomplished using organic or other non aqueous wash solvents which dissolve the oil based ink for removal. This creates a waste solution comprised of the organic or other non aqueous wash solvent and the dissolved oil based contaminant as well as shop towels saturated with such materials.
Such a clean up process gives rise to several sources of both air and water pollution. First, many of the wash solvents needed to acceptably clean the oil based inks or other coating materials contain various volatile organic compounds (VOCs) which are ultimately dispersed into the atmosphere. Secondly, the organic or other non aqueous wash solvents and the dissolved oil contaminants cannot be discharged into the sewer system without extensive and costly processing and separation of the contaminants. In fact, many are considered hazardous wastes whose disposal is tightly controlled. Thirdly, the laundering of shop towels or other clean up aids and materials results in laundry waste water containing finely dispersed particles of the wash solvents and oil based contaminant which are difficult to separate through common filtration. Although technology such as reverse osmosis, distillation or other membrane processes will, in theory, function to separate the finely dispersed wash solvent and contaminant particles from the laundry waste water, such processes are energy intensive and require substantial maintenance and upkeep. As a consequence, few industries are able to economically clean such waste water and most continue to discharge such untreated water to the sewer system. The Clean Water Act and various other state and federal legislation, however, will force those responsible for polluting discharges to treat the effluent and to remove the contaminants regardless of treatment costs. As an alternative, many printers have gone to disposable towels which create disposal problems of their own.
Accordingly, a need exists for a method of cleaning waste water and recovering the contaminants therefrom. More particularly, a need exists for a method of cleaning waste water from various coating processes such as lithographic and other printing processes, paint or varnish application processes and the like. A need also exists for a method of recovering such contaminants from the waste water and from laundering solutions used to clean shop towels and other clean up aids.